That possibility is even more troubling due to recent reports that the country may soon conduct a literally earthshaking underground nuclear-weapons test. How might such a test unfold? And what might we expect afterward?

Recent satellite pictures reveal that the regime of Kim Jong Un is preparing for an underground nuclear bomb blast, according to South Korean intelligence officials. The act would violate U.N. Security Council resolutions and has been preemptively condemned by several countries including the United States.

The images show the nearly complete construction of a new horizontal tunnel at the Punggye-ri nuclear test site, where previous tests were conducted in 2006 and 2009, according to the intelligence sources, cited by the Associated Press.

If tunnels are dug deeply and securely, radiation from a blast can be completely contained, said intelligence analyst John Pike, director of GlobalSecurity.org, a Washington, D.C.-based think tank. If not, explosions can burst through the surface and release contaminants. Or the vaporization of rock in the blast can create subsurface cavities that in turn create surface craters.

In some cases radiation can seep out slowly during the weeks after a test.

But Pike doesn't expect North Koreans to be at too much radiation risk if their government goes forward with nuclear test explosions.

"These are low-yield tests, there aren't very many of them, and they are underground," Pike said.

And if radiation is vented, 80 radionuclide stations worldwide can detect it in the atmosphere and report it to the organization.

Past data from these networks suggest that North Korea's 2006 and 2009 tests were relatively low yield, perhaps the equivalent of the explosive force of a thousand tons of TNT, Pike said—versus about 20,000 tons of TNT for the Hiroshima and Nagasaki bombs.

But the smaller yield doesn't mean that North Korea's weapons program isn't dangerous.

"There are really two views on this matter," Pike explained. "One is that the North Koreans are rather ignorant people, and the low yields indicate that they've had two failures. I may be in the minority, but I don't subscribe to this theory.

"I think that they've had two successful tests, because if you're simply testing the primary stage of a two-stage nuclear bomb, you don't need to test it at full yield for the [secondary] stage."

In the most commonly used nuclear weapon design, hydrogen fusion produces x-rays, which then compress a secondary section of the bomb, causing uranium or plutonium atoms to split and releasing massive amounts of energy.

"So you just need to measure that you transported enough x-rays in the first stage to be able to ignite the secondary stage," Pike said.

Experts believe North Korea currently has enough processed plutonium for half a dozen or so nuclear bombs, which are likely of a relatively large, primitive design—such as might be wheeled into an underground tunnel and detonated, Pike said.

It's unclear whether Kim Jong Un's regime has the capability to produce the type of small nuclear weapon that could fit into a ballistic missile. (See a National Geographic magazine feature on weapons of mass destruction.)

This week's planned rocket test, however, may be meant to publicly demonstrate a step in that direction, Pike said. The secretive nation uncharacteristically invited foreign reporters to eyeball the rocket last week, he pointed out.

"I don't know whether we're going to have a nuclear test, but the South Korean government seems to think that it's a distinct possibility. And it really looks like we're going to have a rocket test—they have been putting on a very big show," Pike said.

"We assume that Kim Jong Un has a need to demonstrate that he is an iron-willed commander by confronting the outside powers."